1. Field of the Invention
The present invention relates to antennas. More specifically, the present invention relates to methods and apparatus for cooling active array radar antennas.
2. Description of the Related Art
Active array antennas allow for electronic steering of a radar beam. Active array antennas include a plurality of antenna sub-arrays commonly referred to as `sticks` comprising 20 to 30 radiating elements. The stick has a finite thickness and a heat exchanger located in a central portion thereof. The heat exchanger is designed to remove heat from circuit components, particularly the transmit/receive (T/R) modules mounted on either side thereof. Unfortunately, the finite length of the heat exchanger imposes some limitations on the system insofar as there is a need to pump coolant therethrough same. In addition, the heat exchanger imposes the more general requirements of fabrication, installation and removal. These considerations limit the optimal size of the exchanger.
An additional problem is due to the physical limitations of the RF (radio frequency) feed located at the back end of the stick. The RF feed is split to distribute power to each radiating element in the array. Each layer of power division requires more physical depth. Unfortunately, the T/R module is conventionally built into a small package. The heat exchanger has walls with a certain thickness. The heat developed within the T/R modules within the high power amplifier (HPA) is considerable due to the high power densities of the modules (on the order of hundreds of watts/sq. cm) and the high associated junction temperatures within the component. Removal of heat is problematic in that the heat must be moved through several thermal/resistant layers. The inherent inefficiencies of the electronic components causes the generation of large amounts of thermal energy, the thermal energy causes an increase in the junction temperatures of the associated components. As the junction temperatures increase, the reliability of the component decreases.
In addition, as the junction temperature is lowered, the overall efficiency of the HPA increases. Hence, as the temperature of the HPA is lowered, the input powered required for a given amount of radiated output power is lowered as well. This is important for aircraft applications where power generation is limited.
Those skilled in the art will appreciate that the critical parameter is area, viz., the footprint of the HPA component verses the thermal resistance of the areas from which heat is being removed.
In addition, conventionally, the components of a T/R module are placed on a substrate mounted within a housing and the T/R module is attached to the heat exchanger. The housing is typically of metallic construction. The module must be physically attached to a sub-array using one of a number of mechanical attachment mechanisms (solder, bolts, etc.). Differing coefficients of thermal expansion between the module and the heat exchanger will cause the expansion and contraction of the heat exchanger to damage the module and/or break a bond line therebetween.
Another consideration has to do with thermal mismatch between a component of a T/R module and its substrate.
Further, lattice spacing is the location of one radiating element relative to the next. Smaller lattice spacing leads to a higher number of T/R modules and a higher antenna output power. Lattice spacing is also driven by antenna operating frequency. Unfortunately, lattice spacings of active array antennas designed in accordance with conventional teachings are constrained based on the projected footprint of the T/R module, notwithstanding the desirability of more flexible lattice spacing arrangements to accommodate frequency.
In summary, there are several problems with the conventional active array antenna design including: high heat density, excessive layers of thermal resistance, physical component assembly, mounting the T/R module to the subarray, lattice spacing, and thermal mismatch.
Hence, there is a need in the art to make smaller HPA components within T/R modules in radar and other electrical systems to achieve smaller denser lattice spacings. There is a further need for a system and technique to more efficiently remove heat from such components. There is also a compelling need to reduce the cost of radar antenna assemblies.